The field of this invention relates to continuous processes for recovery of acid values from hot gaseous mixtures which are obtained by catalytic oxidation of hydrocarbon compounds with a dioxygen containing gas. More particularly, this invention relates to recovery and refining acid values of at least acrylic acid from a gaseous mixture such as is obtainable by gas-phase catalytic oxidation of propylene. Processes of the invention include quenching the gaseous mixture with an aqueous quench liquid to obtain an aqueous solution comprising the acid values; contacting the aqueous solution with an immiscible extraction solvent; and an integrated sequence of distillations and phase separations to recover for recycle organic components of the extraction solvent, and obtain valuable acrylic acid and acetic acid products. Advantageously, according to the invention the immiscible extraction solvent is substantially free of aromatic compounds such as benzene and toluene.
As is well known, most of the commercial acrylic acid is produced from propylene by heterogeneous catalytic oxidation of propylene and/or acrolein in the vapor phase with air and steam. Generally, the two methods for the vapor phase oxidation of propylene are one-stage and two-stage processes. Typically, the process is carried out in two stages giving first acrolein and then acrylic acid. Higher selectivity is possible by using different catalyst compositions and reaction conditions for each of the two stages.
Acrylic acid and esters thereof undergo reactions characteristic of both unsaturated organic compounds and aliphatic carboxylic acids or esters. Acrylic acid and its esters polymerize very easily. Acrylates and acrylic acid are primarily used to prepare emulsion and solution polymers. Emulsion polymerization processes provide high yields of polymers in a form suitable for a variety of applications. Acrylate polymer emulsions are useful as coatings, finishes, and binders for leather, textiles, and paper. Acrylate emulsions are used in the preparation of both interior and exterior paints, floor polishes, and adhesives. Solution polymers of acrylates, frequently with minor concentrations of other monomers, are employed in the preparation of industrial coatings. Polymers of acrylic acid can be used as super-absorbents in disposable diapers, as well as in formulation of superior, reduced-phosphate-level detergents.
Polymerization of acrylic acid and its esters is catalyzed by heat, light, and peroxides and inhibited by compounds such as the monomethyl ether of hydroquinone or hydroquinone, provided oxygen in present. The spontaneous polymerization of acrylic acid is extremely violent.
Commercial production of organic acids is typically accomplished by catalytic oxidation of hydrocarbon compounds having at least one double bond with an oxygen. In particular, production of acrylic acid through gas-phase catalytic oxidation of propylene and/or acrolein is a widely practiced industrial process. This process normally consists of an oxidation step to catalytically oxidize propylene and/or acrolein using molecular oxygen in gaseous phase, a collection step of contacting an aqueous liquid with an acrylic acid-containing gas resulting from the gas-phase catalytic oxidation, and a recovery step to isolate and refine acrylic acid from the aqueous solution of the acrylic acid which is obtained in the collection step.
Unavoidably the acrylic acid-containing gas also contains such co-products as acetaldehyde, formaldehyde, formic acid, and acetic acid, among which acetic acid is in relatively large quantity. For producing high purity acrylic acid, therefore, acetic acid must be removed. Attempts to remove the acetic acid in the acrylic acid by means of distillation, however, tend to induce polymerization of acrylic acid because of the required high distillation temperature. (Boiling point of acetic acid is about 118xc2x0 C.) There is also another problem that the small specific volatility values of acrylic acid and acetic acid render their separation by simple distillation difficult.
Consequently, with the view to isolate and recover high purity acrylic acid from said aqueous acrylic acid solution, that is, to separate acrylic acid from acetic acid and water to recover high purity acrylic acid which is substantially free from acetic acid and water, normally a method of distilling the aqueous acrylic acid solution in an azeotropic separation column in the presence of an azeotropic solvent is adopted.
A process for isolating acrylic acid from aqueous crude acrylic acid by extraction with a ketone and a complex system of distillations is described in U.S. Pat. No. 3,689,541 in the name of Kurt Sennewald, Heinz Erpenbach, Heinz Handte and Winfried Lork. In this process a liquidxe2x80x94liquid extraction of the aqueous stream uses 3,3,5-trimethylcyclohexanone and/or isophorone as the extractant(s). The extract, containing acrylic acid, acetic acid, high boilers and minor proportions of water which are dissolved in the extractant, is introduced into a first distilling column, which is operated under reduced pressure; the bottom product of the first distilling column, containing the high boilers and the extractant(s) is recycled to the extraction; distillate containing acrylic acid, acetic acid, water and minor proportions of extractant(s) is fed to a second distilling column, which is operated under reduced pressure; a mixture of water and extractant(s) is distilled off; the bottom product of the second distilling column, containing acrylic acid and acetic acid is fed to a third distilling column, which is operated under reduced pressure; and acetic acid is separated as a distillate and acrylic acid is recovered as a bottom product.
Later, U.S. Pat. No. 3,781,192 in the name of Kurt Sennewald, Heinz Erpenbach, Heinz Handte, Georg Kohl and Winfried Lork, describes an extractive azeotropic distillation of aqueous crude acrylic acid using 3,3,5-trimethylcyclohexanone as the distillation aid under a pressure between 20 an 80 mm mercury. The bottom product is said to contain acrylic acid, acetic acid and minor proportions of 3,3,5-trimethylcyclohexanone, residual formaldehyde and higher boiling constituents. Acrylic acid contaminated with 3,3,5-trimethylcyclohexanone is recovered as a distillate, which is separated by further distillations.
Several other organic mixtures have been said to be useful to extract acrylic acid from aqueous solution. For example, U.S. Pat. No. 3,962,074 in the name of Wilhelm Karl Schropp describes acrylic acid separation from aqueous solution by extraction of the solution with a mixture of from 1 to 50 parts by weight of butanol and from 1 to 10 parts by weight of butyl acrylate. U.S. Pat. No. 3,968,153 in the name of Tetsuya Ohrui, Yasuhito Sakakibara, Yukinaga Aono, Michio Kato, Hiroshi Takao and Masami Ayano describes extraction of acrylic acid from aqueous solution with methylethylketone containing from 5 to 20 percent by weight of a xylene or ethylbenzene.
U.S. Pat. No. 5,315,037 in the name of Kazuhiko Sakamoto, Hiroaki Tanaka, Masatoshi Ueoka, Yoji Akazawa and Masao Baba assigned to Nippon Shokubai Co., Osaka, Japan states that a process for producing acrylic acid wherein a mixed gas of acrylic acid and by-products produced by catalytic gas phase oxidation of propylene and/or acrolein is contacted with water to obtain an aqueous solution for azeotropic distillation may be improved by using a mixed solvent consisting of A at least one member selected from the group consisting of diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, methyl-tert-butyl ketone and n-propyl acetate and B at least one member selected from the group consisting of toluene, heptane and methylcyclohexane. According to this process the mixing ratio between A and B must be the range of 50:50 to 75:25 of the weight basis. If the amount of A exceeds the above range, the concentration of acetic acid at the bottom of the azeotropic distillation tower is said to become too high. On the contrary, when B is used in larger amounts than the above range, the amount of acrylic acid distilled from the tower top is increased which increase is undesirable.
More recently U.S. Pat. No. 5,910,607 in the name of Yasuyuki Sakakura, Masahiko Yamagishi and Hirochika Hosaka assigned to Mitsubishi Chemical Corp., Tokyo, Japan, states that a process for producing acrylic acid, wherein propylene and/or acrolein is catalytically oxidized with molecular oxygen in a vapor phase and the gas resulting from the oxidation is cooled and/or absorbed in water to form a crude aqueous acrylic acid, followed by azeotropic distillation to remove the water with an entrainer of a boiling point of 80xc2x0 to 130xc2x0 C. on the crude aqueous acrylic acid which may have, upon necessity, undergone removal of aldehydes contained therein to produce acrylic acid purified in that the crude aqueous acrylic acid is substantially dehydrated may be improved by conducting the azeotropic distillation under such conditions that concentrations of the entrainer and water in the bottom product of the azeotropic distillation are, respectively, from 5 percent to 30 percent by weight (entrainer) and no higher than 0.05 percent by weight (water). Where the entrainer is required to azeotropically boil with water, the entrainer is selected from the group consisting of alkyl ester of acetic acid and methyl-isobutyl ketone. Where the entrainer is required to azeotropically boil with both water and acetic acid, the entrainer is selected from the group consisting of aliphatic and aromatic hydrocarbons and isobutyl ether. Preferably concentrations of the entrainer and water in the bottom product of the azeotropic distillation are, respectively, from 6 percent to 15 percent by weight, more preferably 6 to 13 percent (entrainer) and 0.3 percent to 0.05 percent (water).
It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for production of acid values which include at least acrylic acid.
More particularly, it is an object of the present invention to provide an improved method for recovery of acid values from an aqueous solution using an immiscible extraction solvent within an integrated sequence of distillations and phase separations to recover for recycle organic components of the extraction solvent, and obtain valuable acrylic acid and acetic acid products.
Advantageously, the improved processes of the invention should use only immiscible extraction solvents which are substantially free of aromatic compounds such as benzene.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.
Economical processes are disclosed for recovery of acid values from a gaseous mixture formed by catalytic oxidation of propylene in the vapor phase with a dioxygen containing gas and steam. Acid values of acrylic acid and acetic acid produced in the catalytic oxidation reactors is recovered from product gaseous stream as a aqueous solution. Solvent extraction is used to separate most of the water from the aqueous acid solution. Components of the immiscible extraction solvent and low boiling impurities are separated from the acid values by distillation.
Processes of this invention comprise: (a) quenching the gaseous mixture comprising acid values of acrylic acid and acetic acid, steam and one or more non-condensable gas with an aqueous quench liquid to form an aqueous solution comprising acid values; (b) contacting the aqueous solution with an immiscible solvent comprising propyl acetate and a cyclohexane to form an organic extract comprising acid values and a major portion of the propyl acetate, and an aqueous raffinate comprising a minor portion of the propyl acetate; (c) fractionating the organic extract as by distillation to obtain a high boiling fraction substantially free of cyclohexane, preferably less than 1 percent by weight of cyclohexane, and a low boiling fraction comprising cyclohexane and steam; and thereafter (d) fractionating the high boiling fraction to obtain an acrylic acid product substantially free of propyl acetate and a propyl acetate fraction comprising propyl acetate and acetic acid.
Typically, the aqueous solution formed by quenching a gaseous mixture produced by catalytic oxidation of propylene contains up to about 80 percent acrylic acid, preferably from about 25 to about 75 percent of acrylic acid, more preferably from about 30 to about 70 percent of acrylic acid, and most preferably from about 40 to about 60 percent of acrylic acid for best results. Generally, the aqueous solution can also contain up to about 10 percent acetic acid, preferably from about 0.5 to about 7.5 percent of acetic acid, more preferably from about 1.0 to about 5.0 percent of acetic acid, and most preferably at least about 1.5 percent of acetic acid.
In a preferred embodiment of the invention, the aqueous solution comprises from about 30 to about 70 percent of acrylic acid, from about 0.5 to about 7.5 percent of acetic acid and from about 15 to about 65 percent of water based upon the weight of the aqueous solution.
Advantageously, according to the invention the immiscible solvent comprises at least isopropyl acetate and a substantial amount of cyclohexane, in particular no less than about 25 percent of cyclohexane based upon the total weight of isopropyl acetate and a substantial amount cyclohexane. Preferably, according to the invention the immiscible solvent comprises isopropyl acetate and from about 30 to about 80 percent of cyclohexane based upon the total weight of isopropyl acetate and cyclohexane, more preferably about 35 to about 75 percent, most preferred about 45 to about 65 for best results.
Contacting of the aqueous solution with immiscible solvent may employ any of the many known methods for liquid-liquid extraction. In a preferred embodiment of the invention the contacting is carried out using a continuous extraction system with counter-current flow of the aqueous solution and the immiscible solvent. Likewise in preferred embodiments of the invention the integrated sequence of distillations and phase separations are carried out using continuous systems to separate the desired product or products and recover for recycle organic components of the extraction solvent.
An aspect of special significance is condensing at least a portion of the low boiling fraction thereby forming condensate comprising immiscible aqueous and cyclohexane phases, and separating a recovered cyclohexane stream from the aqueous phase. The recovered cyclohexane is advantageously recycled as a component of the immiscible solvent.
In a preferred embodiment of the invention, the acrylic acid product contains less than 0.1 percent by weight of acetic acid. In another embodiments of the invention, the acrylic acid product more preferably contains less than 0.07 percent by weight of acetic acid and most preferably less than 0.05 percent by weight of acetic acid. In yet another preferred embodiment of the invention, the propyl acetate fraction comprises from about 80 to about 93 percent of propyl acetate and from about 2 to about 15 percent of acetic acid based upon the weight of the propyl acetate fraction.
In preferred embodiments of the invention, at least 90 percent of the acrylic acid contained in the aqueous solution is recovered in the acrylic acid product.
One aspect of the invention provides a process for recovery of acid values from a gaseous mixture formed by catalytic oxidation of propylene with a gaseous source of dioxygen which process comprises: (a) quenching the gaseous mixture comprising acid values of acrylic acid and acetic acid, steam and one or more non-condensable gas with an aqueous quench liquid to form an aqueous solution comprising acid values; (b) contacting the aqueous solution with an immiscible solvent comprising propyl acetate and a cyclohexane to form an organic extract comprising acid values and a major portion of the propyl acetate, and an aqueous raffinate comprising a minor portion of the propyl acetate; (c) fractionating the organic extract as by distillation to obtain a high boiling fraction substantially free of cyclohexane and a low boiling fraction comprising cyclohexane and steam; (d) fractionating the high boiling fraction to obtain an acrylic acid product substantially free of propyl acetate and a propyl acetate fraction comprising propyl acetate and acetic acid; and (e) fractionating the propyl acetate fraction to obtain an acetic acid product substantially free of propyl acetate and a recovered propyl acetate fraction. Advantageously the immiscible solvent contains at least a portion of the recovered propyl acetate fraction.
Processes of the invention preferably include condensing at least a portion of the low boiling fraction thereby forming condensate comprising immiscible aqueous and cyclohexane phases, and separating a recovered cyclohexane phase from the aqueous phase. Recovered cyclohexane is advantageously recycled as a component of the immiscible solvent system.
Other embodiments of the invention preferably include forming a stripping tower feed by combining at least a portion of the separated aqueous phase with at least a portion of the aqueous raffinate, and contacting the stripping tower feed with steam to recover solvents and beneficially obtain wastewater suitable for bio-treatment.
Another aspect of the invention provides a process for recovery of acid values from a gaseous mixture formed by catalytic oxidation of propylene with a gaseous source of dioxygen which process comprises: (a) quenching the gaseous mixture comprising acid values of acrylic acid and acetic acid, steam and one or more non-condensable gas with an aqueous quench liquid to form an aqueous solution comprising acid values; (b) contacting at least a portion of the aqueous solution with an immiscible solvent comprising propyl acetate and a cyclohexane to form an organic extract comprising acid values and a major portion of the propyl acetate, and an aqueous raffinate comprising a minor portion of the propyl acetate; (c) fractionating at least a portion of the organic extract as by distillation to obtain a high boiling fraction substantially free of cyclohexane and a low boiling fraction comprising cyclohexane and steam; (d) condensing at least a portion of the low boiling fraction to form a mixture of at least two immiscible liquid phases, and separating from the mixture a recovered cyclohexane phase containing less than about 2 percent by weight of water, preferably less than about 1 percent, more preferably less than 0.5 percent by weight; (e) fractionating the high boiling fraction to obtain an acrylic acid product substantially free of propyl acetate and a propyl acetate fraction comprising propyl acetate and acetic acid; and (f) fractionating the propyl acetate fraction to obtain an acetic acid product substantially free of propyl acetate and a recovered propyl acetate fraction. The immiscible solvent beneficially contains at least a portion of the recovered cyclohexane phase. Advantageously the immiscible solvent contains at least a portion of the recovered propyl acetate fraction and at least a portion of the recovered cyclohexane phase.
Another aspect of special significance is the process wherein the fractionating of the organic extract is carried out in a continuous distillation system using as reflux at least a portion of the recovered cyclohexane.
In preferred embodiments of the invention, the immiscible solvent, comprising propyl acetate and a cyclohexane, contains less than a total of about 10 percent by weight of acrylic acid and acetic acid. More preferably the immiscible solvent contains less than a total of about 5 percent by weight of acrylic acid and acetic acid, and most preferably less than about 2 percent.
For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention.